Two separate studies have documented distinct ancient plague outbreaks, one in prehistoric hunter-gatherer communities in Siberia and another in the Roman-era city of Jerash in modern-day Jordan. The research provides new insights into the timeline, transmission, and demographic impact of Yersinia pestis infections across different historical periods and societies.
Siberian Hunter-Gatherer Outbreaks (circa 5,500 Years Ago)
Discovery and Dating
A study published in Nature reported the detection of Yersinia pestis DNA in 18 out of 46 individuals (approximately 39%) from four hunter-gatherer cemeteries near Lake Baikal in Siberia. The remains were excavated by Russian archaeologists in the 1980s and preserved.
Radiocarbon dating identified two distinct outbreak phases, separated by 4–6 centuries, occurring between approximately 5500 and 4800 calibrated years before present (BP). Genomic analysis indicates the Lake Baikal plague strains diverged before all previously known ancient and modern plague lineages, pushing back the estimated origin of Yersinia pestis as a human pathogen to before ~5,700 years ago.
Demographics and Burial Patterns
Multiple burials suggest simultaneous deaths. One grave contained three children buried together: two half-sisters aged 9–10 and 5–6, and an unrelated boy aged 11–12, all with plague DNA. Another grave contained a pair of cousins or sisters aged 4–9, also all infected.
Mortality patterns indicate unusually high child mortality, peaking at ages 7.5–11 years, which researchers noted is consistent with acute lethal outbreaks and differs from typical hunter-gatherer demographic profiles.
Pathogen Genetics and Transmission
The ancient Yersinia pestis strains from Siberia lack the ymt gene and YpfΦ prophage necessary for flea-borne bubonic transmission. However, they carry the ypm gene encoding a superantigen linked to severe immune overreaction. The ypm variant found is most similar to the modern ypmA form, considered highly virulent, though three amino acid differences exist. The strains also possess ancestral genetic features lost in later plague lineages, representing a transitional stage from Yersinia pseudotuberculosis to Yersinia pestis.
Researchers suggested transmission likely occurred via respiratory droplets or direct contact (pneumonic plague) rather than through flea vectors. The study also proposed that transmission may have involved repeated zoonotic spillovers from rodent reservoirs, such as marmots, which are known plague hosts in the region.
Significance and Limitations
The findings provide the earliest evidence of lethal plague outbreaks outside of large agricultural settlements, indicating that plague could cause severe mortality among small, mobile hunter-gatherer groups. This challenges the theory that plague emergence was primarily driven by the Neolithic agricultural revolution and higher population densities.
Ian Light-Maka (Max Planck Institute for Infection Biology), who was not involved in the study, cautioned that further research is needed to confirm human-to-human transmission. A previous study had found plague DNA in a single hunter-gatherer from Latvia about 5,000 years ago, but no evidence of an outbreak or transmission.
Jerash Plague Outbreak (7th Century CE)
Discovery and Context
A study published in the Journal of Archaeological Science examined victims of a 7th-century plague in the ancient city of Jerash, located in modern-day Jordan. During the outbreak, workshops under the city's Hippodrome were repurposed as a mass grave containing approximately 230 individuals. The research was led by University of South Florida geneticist Rays Jiang, involving experts in archaeology, molecular genetics, anthropology, and chemistry.
The outbreak occurred during the Plague of Justinian, which affected the Mediterranean Basin, West Asia, and Northern Europe between 541 and 750 CE. Jerash, strategically located on a major trade route within the Eastern Roman Empire, experienced multiple plague waves. By around 650 CE, the city's population had decreased to approximately 10,000 residents.
Analysis of Victims
Researchers extracted samples from human teeth discovered during 1980s excavations. They conducted mitochondrial DNA sequencing and stable isotope analysis. Tooth dentine, formed in early childhood, retains isotopic markers that allow reconstruction of an individual's childhood diet.
Analysis of plague microbes from the Jerash bodies indicated extreme similarity across samples, suggesting the bacteria spread rapidly and caused quick fatalities before significant mutation. Yersinia pestis has a high mortality rate, ranging from 60% to 100% without treatment.
The approximately 230 victims in the mass grave included men, women, and children. DNA analysis indicated ancestral ties to diverse regions including central Africa, eastern Europe, and Anatolia. Isotope analysis demonstrated varied childhood diets and geographical origins among the victims. The team noted that these individuals did not appear to be local residents, suggesting they were highly mobile populations such as visiting merchants, foreign workers, or enslaved people.
Preservation and Significance
An earthquake in 659 CE caused the Hippodrome structure to collapse, sealing the mass grave and preserving the remains. Nükhet Varlık, an ancient pandemics expert at Rutgers University not involved in the study, commented that following initial plague waves, cities often required new labor sources, attracting immigrants who then became vulnerable to subsequent outbreaks. The study emphasizes the long-term presence of plague and the importance of managing its spread and containment.